In a vehicle equipped with an electric power steering (xe2x80x9cEPSxe2x80x9d) system, for example, the steering assist torque is provided by an electric motor that is typically coupled to a steering column or shaft. Electric motors and their control circuits and devices generate heat at a rate that increases with operating current, and are thus designed to operate within design limits chosen to prevent damage due to thermal overload. Electric motors used in vehicular systems are subject to a wide range of loads for highly varying periods of time. For example, the electric motor in a vehicular electric power steering system may require only 15 Amperes (xe2x80x9cAxe2x80x9d) of electrical current for long periods of time such as for continuous operation, but may require 75 A of current for shorter periods such as up to about 5 seconds.
One type of approach is exemplified by U.S. Pat. No. 6,166,502 to Pattok et al., which shows a protection method for such a motor that limits a motor control parameter such as the motor operating current to the highest expected value (e.g., 75 A). Unfortunately, this type of approach does not provide different levels of protection for different motor speeds and generally requires that the motor and control circuit be designed to accommodate such loads for a worst-case motor speed. Such a motor and control circuit are greatly over-designed for normal use and thus unacceptably large, heavy, and inefficient for competitive market applications.
It is also known to use thermal sensors such as thermistors to directly measure temperature in a current limiting system. However, such sensors have drawbacks including generally slow response, calibration overhead and difficult placement.
The above described and other features are exemplified by the following Figures and Description in which a vehicular system is disclosed that includes a controller and an electric motor in signal communication with the controller for receiving current and rotating at speed; while the controller includes a first function responsive to a signal indicative of the speed of the electric motor, a filter responsive to a signal indicative of the heat generation rate of the vehicular system, a second function in signal communication with the first function and the filter, and a limit calculation function in signal communication with the second function for providing a motor current limit responsive to the second function; wherein a method for controlling the vehicular system includes receiving a signal indicative of the heat generation rate of the vehicular system, receiving a signal indicative of a speed of the electric motor, providing a scale factor or an overload value in response to the received signal indicative of speed, filtering the received signal indicative of heat generation rate in correspondence with a time constant, processing the filtered signal by executing a multiplication by the scale factor or a comparison with the overload value to produce a signal indicative of compliance with a duty cycle requirement, and calculating a motor current limit in correspondence with the processed signal to thereby protect the system from thermal overload while maximizing its performance.